1. Field of the Invention
The present invention relates to a current source. More particularly, the present invention relates to a current source apparatus for reducing interference with noise.
2. Description of Related Art
Current source is always required in today's electronic products for providing a stable current, for example, a current control oscillator requires a stable current for producing oscillation signals of a specific frequency. An electronic product can operate properly and perform expected functions when the frequency of the oscillation circuit is stable. However, such electronic product cannot be operated properly if the oscillation circuit is not able to provide a stable frequency. Thus, how to provide a stable current source so that the electronic products can operate properly is a very important subject of development.
One of the causes which make a current source unstable is the affection of noises, and is shown in FIG. 1 (the block diagram of a conventional current control oscillator). Referring to FIG. 1, the current source 102 determines the quantity of the current Ib based on the reference voltage Vb. The conventional oscillator 104 determines the frequency of the output oscillation signal Vo thereof based on the quantity of the current Ib. The disadvantage of the conventional oscillator 104 is that the tail current source 102 thereof is working in the saturation region (which is like a common-source amplifier to the noise 106 and a common-gate amplifier to the noise 108). Accordingly, when there is a noise (noise 106 as shown in FIG. 1) occurring at the gate thereof, the noise is amplified, which severely interfere the operation of the oscillator. This current source is equivalent to a common-gate amplifier to the noise produced by the ground GND (denoted as noise 108 in FIG. 1). In other words, when a noise enters the oscillator from the ground terminal, the noise is amplified by the current source working in saturation region, which would also severely interfere the operation of the oscillator. This phenomenon will be described below.
FIG. 2 illustrates a 3-level ring oscillation circuit. Referring to FIG. 2, since the frequency of the oscillator 104 is proportional to the current IDSAT of the tail current source 216, the current IDSAT of the tail current source 216 is changed when a noise (214 or 218) enters the gate or source of the current source, so as to perform frequency modulation to the ring oscillator 104, and the timing response thereof is shown in FIG. 3. Pattern A is the oscillation waveform of an ideal oscillator, and pattern B is the waveform interfered by a noise. It can be understood from pattern B that with noise interference, phase shifts of Δψ1, Δψ2, Δψ3, and Δψ4 occur to the frequency of the oscillator, thus the frequency of the oscillator changes along with the change of time, which may cause phase error (i.e. jitter). Thus, in the conventional circuit, the noise entering from the gate and/or source of the current source may be amplified and which may cause jitters, and the current source in the conventional circuit is very sensitive to the interference of the voltage source VDD, the power supply rejection ratio (PSRR, which shows the capability of preventing noise coupling from power supply) is not ideal. Moreover, the output impedance of the current source in the conventional circuit is low, so that the noise from the ring oscillator itself increases jitter through modulating the current of the current source.